1. Field of the invention
The present invention relates to a magnetic material dispersed type carrier and a process for producing the carrier. The present invention also relates to a two-component type developer for developing electrostatic images, comprises of a toner and a carrier, and an image forming method for developing a latent image by the use of the two-component type developer under application of a bias voltage in a developing zone.
2. Related Background Art
In general, in electrostatic recording apparatus making use of electrophotography, commonly employed is a method in which a photoconductive material such as selenium, OPC (organic photoconductive material) or .alpha.-Si is used in an electrostatic image bearing member, where the electrostatic image bearing member is uniformly charged by various means, thereafter the charged surface of the electrostatic image bearing member is irradiated with a light image to form on its surface an electrostatic latent image corresponding to the light image, and the latent image is converted to a visible image by making toner adhere thereto by magnetic brush development or other developing process.
This developing method makes use of a toner that converts the latent image to a visible image and carrier particles comprising a magnetic material, called a carrier. The carrier imparts the toner to a proper quantity of positive or negative electrostatic charges by triboelectric charging, and also carries the toner on its particle surfaces by the electrostatic attraction force of the triboelectricity.
The developer having such a toner and a carrier is coated on a developing sleeve provided with a magnet in its inside, in a given layer thickness by means of a developer layer thickness control member, and then transported by utilizing a magnetic force, to a developing zone formed between the electrostatic image bearing member described above and the developing sleeve.
A given development bias voltage is applied between the electrostatic image bearing member and the developing sleeve. The toner is fed to the developing zone and performs development on the electrostatic image bearing member.
In general, the carrier that composes the two-component type developer can be roughly grouped into a conductive carrier and an insulative carrier. There are various performances required in these carriers. Particularly important performances are proper chargeability, breakdown strength against applied electric fields, impact resistance, wear resistance, anti-spent properties, developing performance and productivity.
The conductive carrier is usually comprised of oxidized or unoxidized iron powder. A developer comprised of this iron powder carrier, however, has the problem that the triboelectric chargeability to toner is so unstable that fogging may occur on visible images formed using the developer. More specifically, as the developer is used, toner particles adhere to the surfaces of the iron powder carrier particles, so that the electrical resistance of carrier particles increases to lower bias currents, and also to make the triboelectric chargeability unstable, resulting in a lowering of the image density of a visible image formed and an increase of fog.
The insulative carrier is commonly typified by a carrier comprising carrier core particles comprised of a ferromagnetic material such as iron, nickel or ferrite whose surfaces are uniformly coated with an insulating resin. A developer that employs this carrier may little cause the melt-adhesion of toner particles to the carrier surfaces, compared with the case of the conductive carrier, and hence has the advantage that it is suitable particularly for high-speed electrophotographic copying machines in view of its superior durability and long lifetime.
Meanwhile, in either conductive or insulative carriers conventionally available, an increase in true specific gravity results in an increase in the load applied to the developer when the developer is made to have a given layer thickness on the sleeve by means of the developer layer thickness control member. Hence, (a) toner filming, (b) carrier break and (c) deterioration of toner tend to occur during long-term use of the developer, so that the developer tends to deteriorate, accompanied with a deterioration of image quality of developed images. An increase in particle size of the carrier results in an increase in the load applied to the developer and hence the above (a) to (c) is more liable to occur, so that the developer is more subject to deteriorate. It also brings about (d) a poor fine-line reproduction, in other words, a poor developing performance as well known.
Thus, the carriers that tend to cause the above (a) to (c) make it necessary to take troubles to periodically change developers, and are enconomically disadvantageous. Hence, it is necessary to decrease the load applied to the developer or improve impact resistance and anti-spent properties of carriers so that the above (a) to (c) can be prevented to make the lifetime of developers longer.
To cope with the problem on developing performance as noted in the above (d), it is necessary to make the particle size of carriers smaller.
To cope with the problems (a) to (d), a small particle size carrier comprising a binder resin and magnetic particles dispersed therein may be used, as exemplified by a magnetic material dispersed type small particle size carrier prepared by pulverization, as disclosed in Japanese Patent Application Laid-open No. 54-66134, and a magnetic material dispersed type small particle size carrier prepared by polymerization, as disclosed in Japanese Patent Application Laid-open No. 61-9659.
However, unless a large quantity of magnetic material is added to carrier particles, the above magnetic material dispersed type small particle size carriers have so small a saturated magnetization for their particle size that they have a problem of (e) adhesion of carrier to photosensitive members, which may occur during development. This makes it necessary to replenish the developer or provide in an image forming apparatus a mechanism for collecting adhered carriers. Thus, they can not be drastic countermeasures for making the lifetime of developers longer.
In the case when a large quantity of magnetic material is added to the magnetic material dispersed type small particle size carriers, the quantity of the magnetic material increases with respect to the binder resin and hence the impact resistance becomes weak. This tends to cause falling-off of the magnetic material from the carrier when the developer is made to have a given layer thickness on the sleeve by means of the developer layer thickness control member. As a result, the developer tends to deteriorate. Thus, also in this case, they can not be drastic countermeasures for making the lifetime of developers longer.
In addition, in the case when a large quantity of magnetic material is added to the magnetic material dispersed type small particle size carriers, resistance of the carrier decreases because of an increase in the quantity of a magnetic material having a low resistance. As a result, they tend to cause (f) faulty images because of a leak of the bias voltage applied during development.
Thus, these magnetic material dispersed type small particle size carriers are disadvantageous in that they can not be drastic countermeasures for improving developing performance and making the lifetime of developers longer.
A technique in which carrier particles are coated with a resin as disclosed in Japanese Patent Application Laid-open No. 58-21750 can also be another countermeasure. Such a resin-coated carrier can improve anti-spent properties, impact resistance and breakdown strength against applied voltage. Since it can also control charge performance on account of the charge performance of the resin with which the carrier particles are coated, selection of the resin make it possible to impart desired charge to toner.
This resin-coated carrier, however, also has a problem as follows: If the coating resin is in a large quantity to give a carrier with a high resistance, what is called the charge-up of toner tends to occur, which is phenomenon in which electrostatic charge of toner become large in quantity in a low-humidity environment. If the coating resin is in a small quantity, the carrier may have so excessively low a resistance that faulty images caused by a leak of development bias voltage tends to occur. Thus, it is difficult to control its coating weight.
Some coating resins, even those which can be considered to have given a proper resistance when the resistance of a resin-coated carrier is measured, tend to cause faulty images because of a leak-of development bias voltage. Thus, such a resin-coated carrier also has the problem of a difficulty in its control when developing performance is taken into account.
The electrostatic charge of a developer making use of such carriers coated with an insulating resin commonly tends to vary depending on variations in environmental conditions as in a low-temperature low-humidity environment or a high-temperature high-humidity environment. As a result, problems may occur such that the charge-up causes a decrease in image density in a low-temperature low-humidity environment and a decrease in triboelectricity causes fogging or black spots around line images in a high-temperature high-humidity environment.
Thus, under existing circumstances, no carrier having reached a satisfactory level has been discovered in regard to the carrier coated with an insulating resin.
As to a carrier coated with no insulating resin, various attempts have been made. For example, Japanese Patent Application Laid-open No. 62-229256 discloses a carrier comprising ferrite particles to the surfaces of which a water-soluble quaternary ammonium salt is adhered. Use of the water-soluble quaternary ammonium salt, however, has caused the disadvantage that the quaternary ammonium salt on the ferrite particle surfaces is dissolved out or eliminated after a toner has been left standing for a long period of time in a high-temperature high-humidity environment or after running, so that the properties of the particles gradually become close to the properties of untreated ferrite particles. In addition, since the particles are not coated with a resin, the quaternary ammonium salt on the ferrite particle surfaces tends to be eliminated not only after running in a high-temperature high-humidity environment but also after that in a usual environment of normal temperature and normal humidity. Even if the quaternary ammonium salt is not eliminated, as compared with the resin-coated carrier, there has been, after all, a problem that a toner forms a film on the surface of the carrier, i.e., the problem that a toner is so susceptible to the toner-spent that a developer has a short lifetime. In addition, unless the particles are coated with a resin having insulating properties to a certain degree, neither iron oxide powder nor ferrite particles can be suitable for preventing the leak of current in a developing system in which a bias voltage is applied or the adhesion of carrier onto a photosensitive member. Thus, in respect of the durability and anti-spent properties of carriers, no method is presently available which may be superior to the coating of the carrier with an insulating resin.
As discussed above, taking account of the performances required of carriers, the carriers conventionally used still have problems to be settled and no well satisfactory carrier is known at present.
In particular, in magnetic material dispersed type carriers comprising a binder resin and magnetic particles dispersed therein and whose particle surfaces are coated with a resin, no carrier is known at present as to those which can be well satisfactory on the following:
(1) Anti-spent properties. PA0 (2) Impact resistance (preventing carrier from breaking). PA0 (3) Preventing toner from deteriorating. PA0 (4) Developing performance. PA0 (5) Preventing carrier from adhering onto photosensitive members. PA0 (6) Controlling resistance of carrier. PA0 (7) Stabilizing chargeability of toner (making lifetime longer in regard to chargeability). PA0 (8) Stabilizing chargeability of toner against environmental variations. PA0 said carrier core material has a binder resin and fine magnetic material particles dispersed in said binder resin; and PA0 said coating resin material contains at least one member selected from the group consisting of; PA0 said carrier core material has a binder resin and fine magnetic material particles dispersed in said binder resin; and PA0 said coating resin material contains at least one member selected from the group consisting of; PA0 preparaing a coating solution or coating dispersion in which a coating resin material is dissolved or dispersed; said coating resin material containing at least one member selected from the group consisting of; PA0 coating the surface of a carrier core material with the coating solution or coating dispersion thus prepared; said carrier core material having a binder resin and fine magnetic material particles dispersed in said binder resin; and PA0 drying the coated carrier core material to give a carrier. PA0 developing a latent image formed on an electrostatic image bearing member, by the use of a two-component type developer comprising a toner and a carrier, under application of a bias voltage in a developing zone; PA0 said carrier comprising a carrier core material and a coating resin material with which the surface of said carrier core material is coated, wherein; PA0 said carrier core material has a binder resin and fine magnetic material particles dispersed in said binder resin; and PA0 said coating resin material contains at least one member selected from the group consisting of;
In particular, in recent years, there is a tendency that toner particles are made finer from the standpoint of a higher image quality, and hence the electrostatic charges of toner may more greatly vary depending on environmental changes in temperature and humidity. Thus there is the problem that it is more difficult to prevent both the toner scatter and fogging accompanying a decrease in electrostatic charge in a high-humidity environment and the decrease in image density due to the charge-up in a low-humidity environment.